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420SEL 01-30-2001 10:33 PM

I was sitting in my 1986 420SEL the other day, I had just started the car and was waiting for my wife to come out of the house. I decided while waiting to check the air pressure in the tires. As I got to the rear passenger tire I noticed a hissing sound. I listened carefully and am sure it is not from the tire. I was even more sure when I shut the car off and the hissing stopped. Could I have a vacuum leak? Are there any vacuum lines around there? Any other ideas, all locks are functioning fine

JP Toronto

500sec 01-30-2001 10:46 PM

The fuel pump is mounted behind the right rear tire.Every on of those cars i have ever seen has a hissing fuel pump.I hope that is normal,I believe it is

Ashman 01-30-2001 10:55 PM

The hissing should sound more like a buzzing if its the fuel pump. if its actual air pressure hissing noise. look for a vacum leak somewhere.

I have heard you can get a smoke system to put smoke int he vacuum system that will blow smoke out of any leaks.

Remember I'm not a tech though

Alon

Pbalias 01-30-2001 11:35 PM

hissing sound
 
Your hissing sound is an exhaust leak, like a hole in the muffler or resonator or even the pipe itself. I had a 420 that did it and now my 92 400e does it also.



Peter

420SEL 01-31-2001 07:48 AM

I don't think it could be an exhaust leak, I had a new resonator and rear muffler installed just last year. The hissing sound isn't far enough forward of the rear wheel to be the resonator and it isn't far enough on the driver's side of the car to be the muffler. In fact, if I go around to the driver's side by the rear wheel you don't hear the hissing at all.


CloudNine 01-31-2001 03:07 PM

Quote:

Originally posted by 420SEL
I don't think it could be an exhaust leak, I had a new resonator and rear muffler installed just last year. The hissing sound isn't far enough forward of the rear wheel to be the resonator and it isn't far enough on the driver's side of the car to be the muffler. In fact, if I go around to the driver's side by the rear wheel you don't hear the hissing at all.


My car experiences the same hissing sound as described here. It has never been associated with any problem. I would like to know what it is, however.

tryingstar 02-02-2001 03:36 PM

Try locking the doors with the car running and see if it makes any difference. Could be a function of the vacuum locking system. I have a similiar sound and assumed it was exhaust pipe or a flange pinhole crack, haven't been able to isolate it yet. It could be a natural part of the locking system!!

420SEL 02-02-2001 06:20 PM

Tryingstar,

I did the test you mentioned, had my wife in the car with it running and had her lock and unlock the doors several times with no change in the hissing. It must be the fuel pump as others have suggested.


tryingstar 02-05-2001 10:27 AM

Does the sound mean the fuel pump is headed south or normal?

BmwTek 02-05-2001 10:49 AM

my w140 300se has the same thing.. its a pretty audible hissing sound.. i think its my exhaust system.. but not too sure..

BlackE55 02-05-2001 11:41 AM

My 560 SEL does it too. Pretty sure it's the fuel pump.

Back in the early/mid '80's, I recall both an Audi and Saab that were in my family did the same thing. Sounded like a combo. of a hiss/buzz.

The Saab had about 120k miles when we sold it and never replaced the fuel pump. Replaced about everything on the Audi! ;)

joel 02-05-2001 01:58 PM

it is the fuel pump. hissing or buzzing is normal.
happy motoring!!

s60 02-06-2001 04:34 AM

Probably fuel pump. It seems common with Bosch fuel pumps. Had a VW that did the same thing but, can't hear it on my 560SEL

PaulC 02-06-2001 10:53 AM

I'm assuming that you have no driveability problems, particularly no difficulty in starting the engine after the car is sitting several hours or more. If you do, I would wonder about depressurization of the fuel system.

joel 02-06-2001 10:36 PM

paulc:

what do mean by depressuring the fuel system?
how does that work?


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